The 6 year limitation is only on taking Court enforcement action. The debt still exists & can still be chased. If there was a CCJ (& this could have been granted without your partner knowing if she changed address) then the limitation does not apply at all.
But 28 years is ridiculous. Do as New Judge suggests (make sure any contact is in writing & keep a copy & send recorded delivery. If they phone keep a record of date & time of call, name of person calling & what was said).
If they still insist on chasing tell them they have to provide proof of the debt.